I had similar clearance issues on my '83 on that right side, but that was with an aftermarket exhaust. I fixed it by actually tweaking (bending) the headpipe a little. The stock muffler was close but did clear. And like yours, the left side had plenty of clearance. My advice would be to place a thick washer behind the muffler hanger bracket where it attaches to the frame. That should space it out a little bit and give you the clearance you need.